In a cellular communication environment, when a mobile terminal moves from a first cell m a second cell, it is necessary to transfer the mobile terminal's RF communication link with the first cell to the second cell. This operation is commonly called handoff. Most cellular telephone systems in use today divide the available frequency band into channels typically 30 KHz in bandwidth. The cellular system service area is divided geographically into cells of varying sizes. Each cell is controlled by a base station. The available frequency channels the divided into sets with each set usually containing an equal number of channels. The frequency sets the assigned m cells in such a way as m minimize the possibility of co-channel interference. In such a system, when the cell receiver handling the call notices that the receive signal strength from the mobile unit falls below a predetermined threshold value, the cell asks a system controller to determine whether any neighboring cell is receiving the mobile unit's signal with better signal strength than the current cell. The system controller in response to the current cell inquiry sends messages to the neighboring cells with a handoff request which specifies the channel used by the mobile unit. The cells neighboring the current cell employ special scanning receivers which look for the signal from the mobile unit on the specified channel. Should one of the neighboring cells report an adequate signal level to the system controller, the system controller then initiates a handoff from the current cell to the neighboring cell.
In present cellular telephone communication systems, the radio-frequency spectrum allotted is 60 MHz. With such bandwidth, a large number of channels can be dedicated to individual cell-base stations. However, in the Industrial Scientific and Medical (ISM) band, only 26 MHz has been allocated by the Federal Communications Commission (FCC). Further, an implementation of personal communication services (PCS) systems can place a large number of base stations within the same geographic location, e.g., a building. Further, if a frequency-hopping spread spectrum transmission method is utilized to provide PCS systems, the method utilized by the conventional cellular systems of determining The signal strength of the mobile unit cannot be used, since frequency hopping from channel to channel makes it difficult to synchronize with the mobile unit.
The problem then is to determine a method and apparatus which will allow the determination of which cell a mobile unit should be handed off to which will work in the spread spectrum environment and not require the use of reusable dedicated channels.